The use of preforms for manufacturing bottles and other containers is well known. These preforms consist of tubular bodies of plastic material, with a mouth at a first end, a closed bottom at an opposite second end, and a neck adjacent to the mouth, which optionally includes an outer thread and a perimetric annular flange. The preforms are intended for the subsequent manufacture of plastic bottles by blowing the blind tubular part of the preform inside a mold without the neck and the mouth being altered. Two-layered preforms in which at least part of said tubular body is formed by two superimposed layers having different properties are increasingly used. These two-layered preforms are especially designed for packages requiring a layer with barrier property, for example, barrier against light or against certain gases such as O2 and CO2, packages in which an outer layer can be made of a recycled plastic material and an inner layer of a quality plastic material, or packages formed by two layers of plastic materials of a different color for aesthetic purposes.
Two-layered preforms can be manufactured by several techniques, among which injection overmolding is emphasized, which basically consists of first introducing a core in a primary molding cavity for molding a first layer of a primary molding material on the core, and then introducing said core, with said first layer arranged thereon, in an overmolding cavity for molding a second layer of an overmolding material on the first layer to produce the complete two-layered preform. The neck of the two-layered preform can be formed with the material of the first layer or with the material of the second layer by closing and opening a pair of neck half-molds arranged next to the mouth of the primary molding cavity or of the overmolding cavity, respectively. The molds for producing two-layered preforms in many injection overmolding apparatuses comprise multiple primary molding cavities, multiple overinjection cavities, multiple pairs of neck half-molds and multiple grouped cores which are operated together by the injection overmolding apparatus for large productions.
There are different types of injection overmolding apparatuses using different mechanisms and methods for performing the alternate introduction and extraction of the cores from the primary molding cavities and overinjection cavities, the closing and opening of the neck half-molds, the ejection of the finished two-layered preforms, etc. International patent application PCT/ES2006/000047, which is incorporated by reference, describes an example of an apparatus for manufacturing preforms by injection overmolding using multiple cores to be alternately introduced in multiple primary molding cavities and multiple overinjection cavities.
A common problem in the molds of any type of apparatus for manufacturing two-layered preforms by injection overmolding is that the cores, due to their slenderness and to their long cantilevered portion, can be slightly deformed elastically inside the primary molding cavities during the injection of the primary molding material of the first layer due to the differences in the temperature, and therefore in the fluidity, of the plastic material injected in an initial phase and in a final phase of the injection process, and to the random distribution of this material with different degrees of fluidity inside the empty space defined between the primary molding cavity and the core.
The injection nozzle is generally arranged in a closed bottom of the primary molding cavity facing a free end of the core. Thus, if the material injected in the initial phase, which is less hot, and therefore less fluid, due to the time it has been retained in the injection nozzle and adjacent conduit during the movements of position change of the cores, is randomly directed towards a side of the free end of the core, the hotter, and therefore more fluid material injected next and coming from the hot channel will flow more easily and quickly towards the opposite side of the core and towards the area of the mouth of the cavity. The less fluid material will exert a pressure on a first side of the core greater than the pressure exerted by the less fluid material on an opposite second side, and the core will experience a slight elastic deformation towards this second side.
Given that the thicknesses of the layers of the two-layered preform are subject to strict dimensional tolerances, a slight deformation of the core during the injection operation of the first layer results in variations in the thickness of both layers in opposite sides of the two-layered preform which may be unacceptable, taking into account that the thickness of the layers is reduced to significantly small values during the subsequent blow molding process to form the bottle or another container from the two-layered preform.
U.S. Pat. No. 3,301,928, which is incorporated by reference, proposes supporting the core inside the molding cavity by means of a supporting appendage extending from a valve body of an injection valve for injecting molding material arranged in the closed bottom of the cavity. The valve body is axially movable in alignment with the axis of the cavity and the core, and the mentioned supporting appendage has a conical head which is coupled in a conical recess formed at the end of the core whereas the valve is open to inject the molding material into the cavity. During a final phase of the molding operation of the preform, when a small portion of the cavity still has to be filled, the valve body is retracted to allow completely filling the cavity and to then close the passage of molding material into the cavity. However, this arrangement has several drawbacks. On one hand, the incorporation of the injection valve arranged in the closed bottom of the cavity involves a relatively complex construction and operation, and on the other hand, the fact that the core is no longer supported during the final phase of the molding operation does not ensure a perfect alignment of the core in the molding cavity during the entire injection process of the preform.
U.S. Pat. No. 4,086,315, which is incorporated by reference, discloses an apparatus for producing hollow plastic bodies by injection molding a preform on a core introduced in an injection molding cavity and subsequent blowing said preform inside a blow molding cavity. Supports projecting from an area of the closed bottom of the injection cavity make contact with the end of the core to coaxially support it during the entire injection molding operation. These supports leave holes in the preform. The injection molding cavity is furthermore configured to form a protrusion at the axial end of the preform. The core is extracted from the injection molding cavity and then introduced, with the recently formed and still hot preform thereon, in the blow molding cavity for the blow molding operation. In an area of the bottom of the blow molding cavity there is arranged a cylinder with an axially operated piston to press said protrusion of the preform against the end of the core for the purpose of extruding the material of said protrusion and thus close the holes existing in the preform before the blow molding. A drawback of this arrangement is that the incorporation of the cylinder and piston assembly at the end of the blow molding cavity is complex and expensive. Furthermore, if, as usual, the preforms are to be allowed to cool in order to remove them from the core and store them for a subsequent distribution and use in a plurality of blow molding apparatuses, this would limit the use of such preforms with holes to only those blow molding apparatuses which are equipped with the mentioned cylinder and piston assembly to extrude the protrusion and close the holes before the blow molding operation. Furthermore, the apparatus is intended to manufacture monolayer preforms and in the specification is not described nor suggested the way to use it to manufacture two-layered preforms.
International patent application WO 89/07219 A, which is incorporated by reference, describes a process for injection molding a thin-walled, hollow plastic product, wherein a first common mold part is combined with a first complementary mold part to assemble a first mold cavity. Portions of the first complementary mold part contact portions of the first common mold part to rigidly secure mold parts in a position which impedes movement of the mold parts in relation to each other during injection of a first plastic material into the first mold cavity. The first plastic material is shaped such that when it is contained in a second mold cavity, which is formed by combining the first common mold part with a second complementary mold part, it provides stabilizing regions that rigidly secure the mold parts in a position which impedes movement during injection of a second plastic material. However, there is not described nor suggested that the two-layered plastic product obtained by such process is useful to be subsequently blow molded, and in fact the alternate arrangement of monolayer regions of different materials in the hollow plastic product is a serious drawback since the joining areas therebetween are substantially parallel to the pressure direction during a blow molding operation and this involves a risk of said joining areas to be broken.
Patent EP 1 681 239 A1, which is incorporated by reference, describes a multilayer preform obtained by a sequential co-injection process and having inner and outer layers made of PET colored with titanium dioxide to reflect light and a layer made of PET colored with carbon black to absorb light arranged between said inner and outer layers. A drawback related with the sequential co-injection process in comparison with the overmolding process I that the former is not suitable for injecting only two layers forcing to inject an odd number thereof, in general three or five. It neither permits to accurately control the extent of the intermediate layer or layers nor the uniformity in the width of the layers. A lack of precision in the preform can result in irregularities in a container obtained by blow molding the preform, and such irregularities will be the more accentuated the more thinner are the layers of the preform.
WO 95/00325, which is incorporated by reference, discloses a multi-layer plastic preform for use in plastic blow molding including an injection molded inner layer of virgin plastic having an outwardly extending annular flange at an open end of the preform. An injection molded outer layer of post-consumer recycled plastic covers the inner layer and has a thread for securing a closure, and this thread is located toward the closed end of the preform from the annular flange of the inner layer to thereby provide a construction that allows increased use of the recycled plastic. The preform may be provided with an intermediate layer of a gas barrier material when the container to be blow-molded from the preform is to be used to hold gas pressurized liquids such as soft drinks. The need of the mentioned annular flange in the inner layer at an open end of the preform prevents the possibility of making the neck of the preform only with the material of the outer layer.